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Homework Help: Very hard distribution question!

  1. Mar 31, 2010 #1
    The school psychologist was researching the results of a third school and found them to closely fit a normal distribution with a mean completion time of 20.68 minutes and a standard deviation of 5.3 minutes. Unfortuantly, after he had finished calculating the results, he could not read of of the observed values.

    Given there are 125students at the school, and a percentage error of ten percent determine the possible range of values for the observed frequencies, falling between 14.5 and the 19.5 minute interval.


    z(standardised score) = (x-mean/standard dev)
    Percentage error = |o-e|/total frequency * 100.

    I have no clue.. please help me!
  2. jcsd
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